The high level of genetic variation found in human immunodeficiency virus type 1 (HIV-1) populations is of fundamental importance to the emergence of antiretroviral drug resistance and to the challenges encountered in the development of an effective vaccine. Both the high virus mutation rate and the high turnover rate of infected cells drive virus evolution. Recombination is also thought to play a significant role in shaping population diversity. HIV-1 replication and evolution is thought to be responsible for the gradual breakdown of the immune system and is the mechanism of disease progression to AIDS. Despite the fundamental importance of HIV-1 variation to therapy and viral pathogenesis, there are many basic concepts that remain unexplored or poorly understood. One important concept is whether the APOBEC3 proteins can impact HIV-1 variation. The current proposal will explore 3 lines of investigation. First, we propose to examine the interplay between the HIV-1 mutation rate and viral fitness. Although studies have been performed to analyze each, no studies have been performed to date to analyze their relationship to one another. We propose to investigate these aspects of HIV-1 replication by analysis of a small group of nucleoside and non-nucleoside reverse transcriptase inhibitor-resistant viruses that we have discovered have increased or decreased mutation rates compared to wt virus in the presence and absence of APOBEC3 proteins. Second, we propose to explore how adaptive mutations can alter the HIV-1 mutation rate during the selection of antiretroviral drug resistance. We will investigate whether the adaptive selection of improved viral fitness coincides with the selection for viruses that possess a wt mutation rate. We will also assess whether APOBEC3 proteins shape HIV-1 variation. Finally, we will investigate the origins of G-to-A transition mutations in HIV-1 proviruses to determine if frequent, yet sublethal cytosine deamination induced by APOBEC3 proteins impact HIV-1 variation. PUBLIC HEALTH RELEVANCE The high level of variation in HIV-1 populations has led to the emergence of drug resistance and has frustrated efforts in vaccine development. Basic studies of how HIV-1 mutates and evolves will enhance our understanding viral mutagenesis, which could indirectly aid in improving human health.